A hydraulic or pneumatic transmission control system of an automated gearshift transmission usually comprises a pressure supply unit and an actuating member. In the pressure supply unit, a supply pressure is produced in a supply line and held at a specified level. The actuating member comprises control drives usually made as single-action or dual-action control cylinders, such as a clutch control element and a plurality of transmission control elements, whose pressure spaces can be pressurized as necessary via associated control valves which connect them to the supply line, or emptied or depressurized by connecting them to an unpressurized line that leads to a pressure medium sink. In a hydraulic control system, for example, the oil sump can serve as the pressure medium sink, whereas in a pneumatic control system the exhaust air can be discharged through the unpressurized line to the surroundings.
For example, a typical hydraulic transmission control system of an automated gearshift transmission is described in DE 198 49 488 C2. The pressure supply unit of this known transmission control unit comprises an oil pump that can be driven by an electric motor, by means of which hydraulic oil can be conveyed from an oil sump, through a filter element and a one-way valve, into a supply line. To limit the supply pressure in the supply line, a pressure-limiting valve is connected to the supply line, through which surplus hydraulic oil can flow back to the oil sump. To maintain the supply pressure when the oil pump is switched off, a pressure reservoir connected to the supply line is provided. A pressure sensor is also connected to the supply line to detect the supply pressure.
The actuating member of this known transmission control system comprises, on the one hand, a clutch control element made as a single-action control cylinder that can be controlled by an associated clutch control valve, for disengaging and engaging a friction clutch made as a diaphragm spring clutch. On the other hand, the actuating member comprises a selector control element made as a single-action control cylinder that can be controlled by an associated selector control element valve, and a shift control element made as a dual-action control cylinder that can be controlled by two associated shift control valves.
As a special feature, a cutoff valve that can be controlled by the control pressure of the clutch control element is provided, by virtue of which the selector control and shift control elements can only be actuated when the friction clutch is disengaged. To control the friction clutch, a path sensor, which detects the position of the piston rod of the clutch control element, is provided. If there is a defect in the pressure sensor, the supply pressure can no longer be determined and the transmission control system must then immediately switch over to an emergency operating mode, even though the supply pressure in the supply line may still be sufficiently high to enable normal operation at least for a certain time.
On the other hand, from EP 0 933 564 B1 a transmission control system for an automated gearshift transmission is known, in which the actuating device comprises a selector control element and a shift control element each made as a dual-action control cylinder. In a second embodiment of this transmission control system illustrated in FIG. 9 of the document, a respective associated selector control valve and shift control valve are connected immediately upstream from a pressure space of the selector control element and a pressure space of the shift control element. A common main control valve is connected upstream from the respective other pressure space of the selector control and the shift control and from the selector control valve and the shift control valve, by means of which the distribution line concerned can be connected optionally either to the supply line of the pressure supply unit or to an unpressurized line leading to the oil sump.
To the distribution line is connected a pressure sensor whose pressure signal is used, by actuating the control valve, essentially for controlling the selector control and shift control elements. However, by means of this pressure sensor the supply pressure in the supply line can also be determined when the main control valve is fully open to the supply line and the selector control valve and the shift control valve are each in their closed, null positions.
To avoid the associated functional limitations, in DE 199 21 301 A1 a special design of the main control valve is proposed, with which the pressure sensor, this time connected to a secondary line, is connected to the supply line both in the end position of the main control valve when it is fully open to the supply line, and in its end position when it is fully closed relative to the supply line. This enables the supply pressure to be determined and monitored by the pressure sensor even when the distribution line is depressurized, without the selector control valve and the shift control valve having to be in a particular switch position for this. In the modified embodiment, however, the main control valve is substantially more complex and correspondingly more expensive and prone to malfunction.